Molecular aggregation in a hemicyanine dye: modeling by a combined crystallographic and computational approach

Abstract

Molecular aggregation in hemicyanine dye molecules, a problem of fundamental relevance to their linear and nonlinear optical properties, is addressed through a novel approach based on crystal structure investigation combined with semiempirical quantum chemical computations. Crystal structure of the hemicyanine salt N-n-butyl-4-[2-(4-dimethylaminophenyl) ethenyl] pyridinium bromide is investigated. The electronic absorption spectra of this compound in the solid state and in solution are modeled using semiempirical AM1/CI computations on the molecule and its dimers extracted from the crystal lattice; the molecular environment is mimicked by invoking a solvation model. This approach is shown to provide insight into the electronic absorption spectral shifts reported earlier for LB films of amphiphiles based on these chromophores and should prove useful in guiding further efforts at achieving deaggregation in these LB films.